The horn must be grounded to the body. The ground for the horn travels through the steering column to the steering rack and to the chassis. How do you do this?

The positive feed to the horn goes direct from the fuse box to the horn air pump.

The negative (earth feed) comes from the rack clamp being bolted to the chassis.  This earths the steering rack.  This earth is carried across the steering column UJ by the strap which is really just a wire with two eyelets to allow attachment to the UJ bolts.  This strap then earths the steering column into the car interior.   This earth feed goes up the column itself onto the back of the horn press switch in the steering wheel.  It does this via the little spring bit in a plastic tube that goes through the hole in the steering wheel hub.  This little spring bit allows for movement between the wheel and the column so that there is no problems.

The circuit is competed when the horn push is pressed, and that allows the earth feed to continue onto the brass ring, and then down the purple and black wire to the horn air pump.

I think its a very cleaver solution.

PS, when you make up the UJ earth strap connection, make the strap only just long enough to reach between the UJ bolts.  If the wire is too long it will catch on something (I cant remember what it was) and snap.  I discovered this the hard way!

New earth straps for the Steering Column can be made as follows: One strap made from suitable length of flexible copper cable with eye terminals soldered on each end fitted on both clamp bolts on the column U-joint. One strap as above, one end held by the rack grease nipple (Chapman multiple function design), the other by the closest rack to chassis U-clamp nut.